Light emitting diodes (LEDs) are complex solid state semiconductor devices that are specially constructed to release a large number of photons outward thereby converting electrical energy into light. LEDs are generally preferred over incandescent lamps due to their small size, durability, and long life. Additionally, LEDs are preferred over incandescent lamps because less heat is emitted in the conversion of electrical energy into light.
LED lamps are widely used in a variety of applications such as video signals, traffic signal heads, and automotive brake lights. Furthermore, LEDs are found in a variety of consumer electronics products and serve as visible indicators in equipment such as televisions, VCRs, and car electronics. Often LEDs act as a “pilot” light in many electronics appliances to indicate if a circuit is closed. Furthermore, LEDs are popular for board assemblers in the electronics industry as they are quite suitable for high volume assembly using widely available auto-insertion equipment.
During operation of the light emitting device assembly, current passes through the LED via the two terminals of a substrate. The LED will then be illuminated when electrical energy has been converted into light energy. This conversion from electrical to light energy typically will not be 100% efficient, and as a result, some heat will be produced in this conversion process.
When the LED begins to conduct, the assembly undergoes a gradual increase in voltage while the current flow rapidly increases. A forward voltage of approximately 1.9 V usually is necessary to produce a forward current, in the direction of its greatest conduction, of approximately 20 mA. Naturally, the voltage at 20 mA is dependent on the type of semiconductor material that is used to fabricate the LED. For example, an aluminum indium gallium phosphide (AlInGaP) LED needs 1.9 V while an indium gallium nitride (InGaN) LED needs 3.4 V. The forward-biasing voltage causes electrons and holes to enter the depletion region and recombine; the external energy provided by this voltage excites electrons at the conduction band, which then fall to the valence band and recombine with the holes, resulting in light radiation in the visible spectrum.
In order to maximize the number of photons emitted by the LED and thus produce more light, the forward current needs to be maximized. As the forward current increases, more electrons will be excited at the conduction band and more photons will be emitted. The light that is emitted by the LED is directly proportional to the current running through the LED. Even with the linearity of light output and current, the emission curve of the LED experiences a drop in efficiency as the operating temperature of the LED increases due to the self-heating of the LED as it is driven to higher currents as depicted in FIG. 2; furthermore, the conversion efficiency also experiences a drop. Accordingly, this process suffers from the disadvantage that as the drive current increases, more heat is produced. Light performance of a LED is of greatest importance, and in order to maintain the light performance, the heat produced must then be removed.
In the LED industry, thermal resistance is used to designate the heat transfer ability of the device as a whole. For example, a 5 mm through-hole lamp typically has a thermal resistance (junction to pin) of 240° C./W, indicating that if the LED dissipates 1 W of heat, there is a temperature difference between the LED and the point in the pin of 240° C. Thus, it would be desirable to improve the thermal resistance in order to maximize heat extraction in these devices.
As a specific example of implementation of the prior art design, a LED chip is affixed to a substrate, usually a leadframe, using a type of electrically conductive adhesive. The substrate is comprised of two terminals extending in the axial direction, a portion of the first terminal being composed of a cavity and a straight element. An electrical connection is then made between the LED chip and a second terminal of the substrate. An optically clear epoxy encapsulates the assembly.
As a further example of implementation of the prior art design, U.S. Pat. No. 6,518,600 discloses dual encapsulation layers for an LED. The first encapsulation layer is transparent to light or radiation, and the second encapsulation layer has high thermal conductivity in order to decrease the operating temperature of the LED. Although the dual encapsulation layers will improve thermal conductivity to a degree, there is a need to further enhance heat extraction from the LED.
Heat removal typically is limited to conduction down the first terminal of the substrate. Since the vertical cross-sectional area of the first portion of the first terminal typically does not comprise more than 30% of the total vertical cross-sectional area of the encapsulated assembly, it generally is difficult to conduct much heat away from the LED. Accordingly, in the past, these devices have been limited to a maximum forward current of approximately 50 mA or less (usually in the range of 10-40 mA). Thus, these parameters result in a less efficient and less high performing LED than is desirable. Since LEDs are becoming widely used, there is a need to extend the performance of these devices by finding a way to enhance heat extraction from the LED.